Wastewater Treatment Methods Questions and Answers

Answer: A

Dissolved substances are removed in the Activated sludge unit.

Activated sludge is the biological (secondary) treatment step. An aerated tank holds a dense, mixed population of bacteria and protozoa that feed on the dissolved and colloidal organic matter in the sewage, converting it into carbon dioxide, water and new cells. That is how the soluble BOD, which no settling tank can catch, is removed.

The secondary clarifier only settles out the microbial flocs produced in the aeration tank. The grit chamber is a preliminary unit that removes sand and gravel. Sludge digestion treats the separated sludge, not the liquid flow.

Answer: A

The organisms in an oxidation pond are Bacteria and Algae.

An oxidation (stabilisation) pond works by a partnership. Bacteria, which are prokaryotes, break down the organic matter of the sewage, releasing carbon dioxide, ammonia and phosphate. Algae, which are eukaryotes, use those nutrients and sunlight for photosynthesis and release oxygen, which the bacteria need to keep oxidising the waste.

The other pairs have no prokaryote at all: mushrooms are fungi, mosses are plants, and lichens are fungus-alga partnerships. None of them is the working population of a sewage pond.

Answer: A

The suspended growth process is the Activated sludge process.

In a suspended growth system the microorganisms are kept floating in the wastewater as flocs, mixed and aerated in a tank. The activated sludge process works this way, and part of the settled sludge is returned to keep the microbial population high.

The other three are attached growth (fixed-film) systems, where the microbes grow as a biofilm on a surface. In a trickling filter it is the stones or plastic media over which the sewage trickles, in a rotating biological contactor it is a set of rotating discs, and in an anaerobic filter it is packing media in a closed reactor.

Answer: A

Methane is obtained with a sludge digester.

In anaerobic sludge digestion, the organic matter in sewage sludge is broken down in closed tanks without oxygen. Acid-forming bacteria and then methanogens convert it to biogas, typically 60 to 70% methane, which can be burnt to heat the digester or generate power.

Vacuum filtration and drying beds only dewater sludge and yield no gas. Composting is aerobic, so its carbon leaves mainly as carbon dioxide, not methane.

Answer: D

The answer is All of the above.

Grit chambers in preliminary treatment let heavy inorganic particles such as sand, gravel and cinders settle, while the lighter organic solids pass on (C).

Removing this abrasive material protects pumps and other mechanical equipment from wear (A) and stops it from depositing in channels and pipes, where it could cause plugging (B; the paper prints 'pip'). It also keeps grit from building up in digesters and clarifiers. As all three statements describe grit removal, no single option is complete.

Answer: C

Activated sludge depends on chemoheterotrophs.

The activated sludge process uses an aerated mass of microbes to oxidise the organic matter in sewage. The dominant bacteria, such as Zoogloea and Pseudomonas, obtain both energy and carbon from organic compounds by chemical oxidation, which makes them chemoheterotrophs.

Chemoautotrophs, such as nitrifying bacteria, are present but take carbon from CO₂ and play a smaller part. Photoautotrophs and photoheterotrophs need light, which is scarce in the turbid aeration tank.

Answer: C

The process that does not soften water is coagulation using aluminium sulfate.

Hardness is caused by dissolved calcium and magnesium ions. Coagulation with alum makes suspended particles clump together so they can settle; it removes turbidity, not dissolved hardness.

The others remove calcium and magnesium: reverse osmosis forces water through a membrane that holds back the ions, ion exchange resins swap them for sodium or hydrogen ions, and Clark's process adds lime to precipitate temporary hardness as calcium carbonate.

Answer: B

The correct sequence is Screening → Coagulation → Flocculation → Sedimentation → Filtration → Disinfection.

1. Screening removes floating debris, leaves and large objects.
2. Coagulation: a coagulant such as alum is mixed in rapidly to neutralise the charge on fine colloids.
3. Flocculation: gentle stirring lets the destabilised particles grow into flocs.
4. Sedimentation: the flocs settle out in a clarifier.
5. Filtration through sand removes the remaining fine particles.
6. Disinfection, usually chlorination, kills pathogens just before supply.

Disinfection must come last, so sequences starting with it are wrong. Flocculation cannot come before coagulation, and filtration before sedimentation would clog the filters quickly.